|  |  | Overview
Networks are complex environments involving multiple media, multiple protocols, and interconnections to networks outside an organization's central office. Well-designed and carefully installed networks can reduce the problems associated with growth as a networking environment evolves. Designing, building, and maintaining a network can be a challenging task. Even a small network that consists of only fifty nodes can pose complex problems that lead to unpredictable results. Large networks that feature thousands of nodes can pose even more complex problems. Despite improvements in equipment performance and media capabilities, designing and building a network is difficult. This chapter provides a review of the Open System Interconnection (OSI) reference model and an overview of network planning and design considerations related to routing. Using the OSI reference model as a guide for network design can facilitate changes. Using the OSI reference model as a hierarchical structure for network design allows you to design networks in layers. The OSI reference model is at the heart of building and designing networks, with every layer performing a specific task to promote data communications. In this semester, Layer 1 through Layer 4 are the focus. These four layers define the following: The type and speed of LAN and WAN media to be implemented How data is sent across the media The type of addressing schemes used How data will be reliably sent across the network and how flow control will be accomplished The type of routing protocol implemented